Reciprocating compressors typically include one or more pistons that “reciprocate” within a closed cylinder. They are commonly used for a wide range of applications that include, but are not limited to, the pressurization and transport of natural gas, air, pure gases, and/or gas mixtures through systems that are used for gas transmission, distribution, injection, storage, processing, refining, oil production, refrigeration, air separation, utility, and other industrial and commercial processes. Reciprocating compressors typically draw a fixed mass of gaseous fluid from a suction pipe and, a fraction of a second later, compress and transfer the intake fluid into a discharge pipe.
Reciprocating compressors can produce complex cyclic pressure waves, commonly referred to as pulsation frequencies, which depend upon the operating speed, the gas thermodynamic properties, pressure and temperature, and the design of the gas compression system. For example, reciprocating compressors will typically produce a one or two times the compressor operating speed pulsation frequency, depending upon their design as a single or a double acting compressor, respectively. In addition, the compressor cylinders and piping systems have individual acoustic resonance frequencies. These pressure waves travel through the often complex network of connected pipes, pressure vessels, separators, coolers and other system elements. They can travel for many miles until they are attenuated or damped by friction or other means that reduce the dynamic variation of the pressure.
Over time, the magnitude of the pulsations may excite system mechanical natural frequencies, overstress system elements and piping, interfere with meter measurements, adversely affect compressor cylinder performance, and affect the thermodynamic performance as well as the reliability and structural integrity of the reciprocating compressor and its piping system. Therefore, effective reduction and control of the pressure and flow pulsations generated by reciprocating compressors is necessary to prevent damaging shaking forces and stresses in system piping and pressure vessels, as well as to prevent detrimental time-variant suction and discharge pressures at the compressor cylinder flanges.
In order to reduce, attenuate and/or control the amplitude of system-damaging pressure pulsations upstream and downstream of a reciprocating compressor, it has previously been customary to use a system of expansion volume bottles, choke tubes, orifices, baffles, chambers, etc. that are installed at specific locations in the system piping. These prior art pulsation attenuation devices can be used singly or in combination to dampen the pressure waves and reduce the resulting forces to acceptable levels. However, these devices typically accomplish pulsation attenuation by adding resistance to the system. This added resistance causes system pressure losses both upstream and downstream of the compressor cylinders. When using prior art pulsation attenuation devices, the resulting pressure drop typically increases as the frequency of the pulsation increases. These pressure losses add to the work that must be done by the compressor to move fluid from the suction pipe to the discharge pipe. Although these pressure losses reduce the overall system efficiency, this has been the accepted state-of-the-art technology for reciprocating compressor systems for more than half a century, and the efficiency, power and energy penalty has been tolerated in order to improve the mechanical reliability and integrity of the system.
Although improvements in system modeling have sometimes showed improved results using traditional pulsation attenuation devices, the problem of high system pressure losses continues to be a persistent issue, especially on high flow, low pressure ratio reciprocating compressors. The problem is more serious as energy costs and environmental regulations mandate improvements in system efficiency. Over the past three decades, it has become common to operate large reciprocating compressors at speeds ranging from 600 to 1,200 rpm, instead of the conventional low-speed (200 to 360 rpm) compressors. High-flow, low ratio reciprocating compressors (generally operating at about 600 to 1,000 rpm, with pressure ratios in the range of about 1.1 to 1.6) can experience large system pressure drops with the addition of current pulsation dampeners. In some cases, system pressure drops have resulted in power losses exceeding 15 to 20%, and have been known to be as high as 30%.
As these larger high-speed reciprocating compressors have been increasingly used, pressure losses caused by the addition of traditional pulsation attenuation systems have become more problematic, due to the higher frequency pulsations that must be damped. Significant pressure losses and increased power consumption have also been encountered on high-speed compressors in some higher ratio applications, especially when a wide range of operating conditions is required.
Therefore, the need for a new technology and method for controlling reciprocating compressor pulsations has been increasingly apparent. Further, a natural gas pipeline or other system that addresses pulsation by attenuating pulses at various positions along the pipeline without significantly affecting efficiency of the system is very desirable. Such technology is also desirable in pipeline systems at metering locations, where pulsations interfere with measurement accuracy and reduce flow measurement instrumentation reliability. Such a new technology, finite amplitude wave simulation, has been successfully applied to 2-stroke and 4-stroke engines to increase specific output and reduce exhaust emissions and noise. Advanced computational technology exists for modeling and designing effective engine tuning systems for high-performance racing, recreational and industrial engine applications. However, all of the aforementioned applications of finite amplitude wave simulation technology have typically been applied (with air or low-pressure mixtures of air and fuel) at pressure levels at or near atmospheric pressure, and at no more than about 3 atmospheres of pressure.
Recently, a new technology that involves cancellation of pulsations, rather than dampening, has been used with high flow, low ratio reciprocating compressor systems. The theory behind pulsation attenuation is premised on the idea that, by properly phasing the cylinders of a reciprocating compressor and/or properly choosing the lengths and diameters of pipes in fluid communication with the compressor, outward bound pulsations can be attenuated, and inward bound pulsations can be used to improve the performance of the compressor.
Published U.S. patent application Nos. 2009/0038684 and 2010/0111713, both of which are incorporated herein by reference in their entirety, disclose this technology, which utilizes finite amplitude wave simulation technology or other simulation means, and includes a network of branches of pipes, called a “tuned delay loop” or “tuned loop,” located upstream and downstream of a reciprocating compressor.
The tuned loops typically split the main pipe section into two parts, which are then subsequently joined. Typically the two wave parts travel different distances and are then recombined at a later point. The different distances will time delay or phase shift the two wave parts. This time/phase shift will cancel frequency components that are present in the repeating wave. The difference in length of the two paths can be “tuned” to the frequency of a wave to dramatically reduce the pulsation in the pipe. When the difference in length is tuned to the rotating speed (rpm's) of a reciprocating compressor, the pulsations will be substantially reduced without a significant pressure loss. In addition, the pulsations can be time phased at the cylinder suction and discharge to reduce the amount of pumping work that is required. This is accomplished by reflecting a pressure wave to increase the pressure at the cylinder discharge during the suction event and similarly reflecting a pressure wave to decrease the pressure at the cylinder discharge during the discharge event.
In light of this new pulsation attenuation technology, a need exists for a mechanical element that enables and simplifies the fabrication and reduces the cost of the individual tuned loops. There also exists a need to provide the precise internal transition geometry, structural integrity, safety and pressure containment of any gas, including explosive, hazardous, lethal, or toxic gases, required at the divergence and convergence points of the tuned loops or branches. Therefore, a primary object of the present invention is to provide a branching device for creating a junction within pulsation attenuation technology.